Orthogonal frequency division multiplexing is a multi-carrier transmission technique that uses orthogonal subcarriers to transmit information within an available spectrum. Since the subcarriers are orthogonal to one another, they may be spaced much more closely together within the available spectrum than, for example, the individual channels in a conventional frequency division multiplexing (“FDM”) system.
In an OFDM system, the subcarriers may be modulated with a low-rate data stream before transmission. It is advantageous to transmit a number of low-rate data streams in parallel instead of a single high-rate stream since low symbol rate schemes suffer less from intersymbol interference (“ISI”) caused by multipath fading. For this reason, many modem digital communications systems are turning to the OFDM as a modulation scheme for signals that need to survive in environments having multipath or strong interference. Many transmission standards have already adopted the OFDM system, including the IEEE 802.11a standard, the Digital Video Broadcasting Terrestrial (“DVB-T”), the Digital Audio Broadcast (“DAB”), and the Digital Television Broadcast (“T-DMB”).
At the transmitter side for OFDM signals, the data is encoded, interleaved, and modulated to form data symbols. Overhead information is added, such as pilot symbols. The symbols (data plus overhead) are organized into OFDM symbols. Each symbol is allocated to represent a component of a different orthogonal frequency. An inverse fast Fourier transform (“IFFT”) is applied to the OFDM symbol to generate time samples of a signal. Cyclic extensions are then added to the signal and passed through a digital-to-analog converter. Finally, the transmitter transmits the signal to a receiver along a channel.
When the receiver receives the signal, the inverse operations are performed. The received signal is passed through an analog-to-digital converter, and timing information is then determined. The cyclic extensions are removed from the signal. The receiver performs a FFT on the received signal to recover the frequency components of the signal, that is, the data symbols. Error correction may be applied to the data symbols to compensate for variations in phase and amplitude caused during the propagation of the signal along the channel. The data symbols are then demodulated, de-interleaved, and decoded, to yield the transmitted data.
The variations in phase and amplitude resulting from propagation along the channel are referred to as the channel response. The channel response is usually frequency and time dependent. If the receiver can determine the channel response, the received signal can be corrected to compensate for the channel degradation. The determination of the channel response is called channel estimation. The inclusion of pilot symbols in each OFDM symbol and/or in each subcarrier allows the receiver to carry out channel estimation. The pilot symbols are transmitted with a value known to the receiver. When the receiver receives the OFDM symbol, the receiver compares the received value of the pilot symbols with the known transmitted value of the pilot symbols to estimate the channel response.
The pilot symbols are overhead, and should be as few in number as possible in order to maximize the transmission rate of data symbols. Since the channel response can vary with time and with frequency, the pilot symbols are scattered amongst the data symbols to provide as complete a range as possible of channel response over time and frequency. The set of frequencies and times at which pilot symbols are inserted is referred to as a pilot pattern. FIG. 1 illustrates a pilot distribution pattern of the DVB-T system. Referring to FIG. 1, the pilot symbols are scattered amongst the data carrier symbols along the time domain (y-axis) and the frequency domain (x-axis) forming a grid to enable two-dimensional interpolation. The solid black circles represent the pilot symbols, and the empty circles represent the data carrier symbols. The variable K represents the number of subcarriers in the frequency domain. As an example, for DVB-T systems, in the 2K mode, there are a total of 1705 subcarriers; and in the 8K mode there are a total of 6817 subcarriers.
FIG. 2 illustrates the conventional approach for channel estimation through interpolation in the time domain (y-axis) and in the frequency domain (x-axis). Referring to FIG. 2, the conventional methods for channel estimation involve using a 2-D Weiner filter or other 2-D filters to interpolate a current data carrier by using known data carriers, also known as pilot symbols, surrounding that current data carrier.
In order to estimate the channel response for a current data carrier, future and past frames must be stored for this symbol up to a certain number of symbols, where that number depends on the amount of Doppler shift and the amount of acceptable error in the channel estimation. Referring to FIG. 2, symbol 210 is the current symbol that is being interpolated. In order to interpolate this data carrier, symbol 210, generally 12 scattered pilot symbols must be stored. These 12 scattered pilot symbols are illustrated by the lines connecting symbol 210 and the respective pilot symbols.
For the 8K mode in the DVB-T, 2730 bytes are needed to be stored for one scattered pilot symbol. Therefore, using 12 scattered pilot symbols for channel estimation requires 32.769 Kb of memory. Fewer than 12 pilot symbols may be used for channel estimation, but this comes at the cost of performance degradation. However, the minimum number of pilot symbols needed for interpolation is three. Thus in terms of memory requirements, 8190 bytes for the 8k mode and 2048 bytes in the 2K mode are needed to store three pilot symbols.
Therefore, it is desirable to provide methods for channel estimation where the amount of memory used for channel estimation can be reduced.